延遲載入，玩過hibernate的都知道那玩意叫懶載入。

在最新官方MyBatis文件裡，有上面這2個屬性，一個是延遲載入，一個是分層載入。

lazyLoadingEnabled 預設值為false，那麼在有級聯關係的resultMap裡，查詢後會加載出所有的級聯關係，當然有時候我們並不需要這些所有的時候，我們就可以應用到延遲載入給我們帶來的好處了。

aggressiveLazyLoading預設值是true，這裡我稱之為分層載入，大概意思是如果它為true，那麼當我使用了延遲載入，要麼所有級聯都不記載，要麼如果我載入一個，其他都得載入

fetchType是可以註明在association 和 collection裡的，選值為eager和lazy，就是為了方便我們結合aggressiveLazyLoading（false）來配合使用的，讓延遲載入發揮到極致，即只加載我需要的！

以下是我做得一個demo

這是實驗要用到的3個表模型

以下是resultMap對應的POJO

package Model;

import java.io.Serializable;
import java.util.List;

public class Student implements Serializable{
	private Integer id;
	private String studentName;
	private String studentAge;
	private List studentHealthCards;
	private ParentOfStudent parentOfStudent;
	public List getStudentHealthCards() {
		return studentHealthCards;
	}
	public void setStudentHealthCards(List studentHealthCards) {
		this.studentHealthCards = studentHealthCards;
	}
	public ParentOfStudent getParentOfStudent() {
		return parentOfStudent;
	}
	public void setParentOfStudent(ParentOfStudent parentOfStudent) {
		this.parentOfStudent = parentOfStudent;
	}
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getStudentName() {
		return studentName;
	}
	public void setStudentName(String studentName) {
		this.studentName = studentName;
	}
	public String getStudentAge() {
		return studentAge;
	}
	public void setStudentAge(String studentAge) {
		this.studentAge = studentAge;
	}
	@Override
	public String toString() {
		return "Student [id=" + id + ", studentName=" + studentName + ", studentAge=" + studentAge
				+ ", studentHealthCards=" + studentHealthCards + ", parentOfStudent=" + parentOfStudent + "]";
	}
	
}



package Model;

import java.io.Serializable;

public class StudentHealthCard implements Serializable{
	private Integer id;
	private Integer stu_id;
	private String name;
	private String message;
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public Integer getStu_id() {
		return stu_id;
	}
	public void setStu_id(Integer stu_id) {
		this.stu_id = stu_id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getMessage() {
		return message;
	}
	public void setMessage(String message) {
		this.message = message;
	}
	@Override
	public String toString() {
		return "StudentHealthCard [id=" + id + ", stu_id=" + stu_id + ", name=" + name + ", message=" + message + "]";
	}
	
}


package Model;

import java.io.Serializable;

public class ParentOfStudent implements Serializable{
	private Integer id;
	private Integer stu_id;
	private String name;
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public Integer getStu_id() {
		return stu_id;
	}
	public void setStu_id(Integer stu_id) {
		this.stu_id = stu_id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	@Override
	public String toString() {
		return "ParentOfStudent [id=" + id + ", stu_id=" + stu_id + ", name=" + name + "]";
	}
	
}
 

<resultMap type="Model.Student" id="stu">
		  	<id property="id" column="id"/>
		  	<result property="studentName" column="student_name"/>
		  	<result property="studentAge" column="student_age"/>
		  	<association property="parentOfStudent" column="id" select="selectOneParentOfStudent"></association>
		  	<collection property="studentHealthCards" column="id" ofType="Model.StudentHealthCard" select="selectOneStudentHealthCard" fetchType="eager"></collection>
		  </resultMap>
		  
		  <select id="selectOneStudent" resultMap="stu">
				select * from student where id = #{id}
		  </select>
		  
		  <select id="selectOneParentOfStudent" resultType="Model.ParentOfStudent">
		  	select * from parentofstudent where stu_id = #{id}
		  </select>
		  
		  <select id="selectOneStudentHealthCard" resultType="Model.StudentHealthCard">
		  	select * from studenthealthcard where stu_id = #{id}
		  </select>
 

情況1：開啟延遲載入，預設分層載入，不開啟區域性載入

執行語句 Student student = sm.selectOneStudent(1);

以下是執行結果：

執行語句：Student student = sm.selectOneStudent(1);
student.getParentOfStudent();

執行結果：

這就是預設分層載入的後果，好的那麼現在我把分層載入設定為false

即情況2：開啟延遲載入，分層載入false，不適用區域性載入

執行以上相同語句：

好了 3條sql變成了2條

情況3：不列出來了，就是使用fetchType的情況下，可以指明及時在延遲載入情況下也可以立即載入某個級聯關係